Distribution transformer cores are manufactured using a continuous ferromagnetic amorphous metal strip, which is wound into a toroid or formed into a rectangular shape from cut strips. Amorphous strip is neither magnetically oriented nor stress relieved as delivered. The material is highly stressed from the casting process. The finished cores must be annealed in the presence of a magnetic field to develop the best magnetic properties and remove the internal stress induced during the manufacturing process. Annealing must be performed at a temperature under 400.degree. C. because at higher temperatures crystallization occurs, which destroys the magnetic properties of the strip. Until now, the strip has not been insulated because no insulation material has been found which has the necessary properties, and because, at widths under about 4 inches, the metal has a sufficiently high resistivity to perform satisfactorily without insulation. However, the industry is presently moving to widths in excess of 4 inches, and it has therefore become necessary to find a satisfactory insulation for the strip.
Because the strip is more brittle and more difficult to handle after it has been annealed, and because the application of prior art coatings after annealing would induce stresses into the strip which would impair its magnetic properties, the strip must be insulated before it is annealed. Since annealing is performed at temperatures between about 350.degree. and 400.degree. C., the insulation must be able to withstand these high temperatures. In addition, the insulation must be extremely thin, e.g., less than 0.01 mils per side, as thicker insulation would increase the space factor and decrease the magnetic properties of the core. While organic coatings have been tried, the coefficients of thermal expansion of organic coatings differ significantly from the coefficient of thermal expansion of the strip, which induces stresses in the strip and thereby reduces the magnetic properties of the strip. Also, very few organic coatings can withstand the annealing temperatures and can be applied extremely thinly. In an alternative approach, a manufacturer of the strip recently attempted to insulate it with a fine calcium silicate dust. However, this did not prove to be entirely satisfactory because the dust was unstable, provided poor insulation, and was easily inadvertently removed during the manufacture of the core.